Our work has shown that the nitrosoureas CNU and CHLZ produce much greater differential toxicities in a normal/transformed human cell system than do other closely related analogs such as CCNU, BCNU, MNU, and FCNU. We are currently attempting to discern the molecular basis for such differential toxicity. We have determined that thymidine is highly mutagenic to mammalian cells and have shown that the mechanism is due to nucleotide base imbalance, since the effects are reversed by deoxycytidine. We have shown that mutation by nitrosoureas in mammalian cells is correlated with DNA single-strand breaks but that cytotoxicity is correlated with DNA-crosslinks. We have developed a new and very sensitive assay for DNA double-strand breaks and have begun to study repair and production of double-strand breaks after both X-ray and bleomycin. Sister chromated exchanges have been shown not to correlate with either mutations or toxicity, but only with DNA single-strand breaks. We have shown that radical scavengers will reduce the production of DNA strand breaks by sunlight and fluorescent light. Thus, both light sources may produce intracellular radicals which cause DNA strand breaks.